1. Field of the Invention
The present invention relates to a method for manufacturing a magnetic recording medium used in a hard disk device or the like, the magnetic recording medium, and a magnetic recording-reproducing apparatus.
2. Background Art
Recently, the application range of magnetic recording apparatuses, such as magnetic disk devices, flexible disk devices, magnetic tape devices or the like, has widened to a great extent, and thus their importance is increasing. In addition, attempts to greatly enhance the recording density of the magnetic recording medium used in these devices are ongoing.
In particular, since the introduction of an MR head and PRML technique, the increased surface recording density growth has become more and more intense. Recently, since the introduction of a GMR head and a TMR head, surface recording density has continuously increased at the pace of approximately 50% a year.
The magnetic recording medium is required to achieve the higher recording density in the future. For this reason, it is necessary to heighten the coercivity, the signal/noise ratio (SNR) and resolution of a magnetic recording layer. Further, recently, efforts to increase the line recording density and the track density so as to increase the surface recording density continue.
In the latest magnetic recording apparatuses, the track density reaches 110 kTPI. However, a problem is liable to occur in that as the track density is increased, pieces of magnetic recording information interfere with each other between adjacent tracks and a magnetization transition region of a boundary area becomes a noise source, thereby deteriorating the SNR. This causes a bit error rate to deteriorate, which is an obstacle to increasing the recording density.
In order to increase the surface recording density, it is necessary to make the size of the respective recording bits on the magnetic recording medium fine, and to provide the respective recording bits with saturation magnetization and a magnetic film thickness as much as possible. However, there is a problem in that if the recording bit is miniaturized, magnetization minimizing volume per bit is decreased, so that recording data disappears due to magnetization reversal caused by heat fluctuation.
Further, since the distance between the tracks is contiguous, the magnetic recording apparatus requires a track servo technique of very high precision, and a method for executing the recording widely and the reproducing more narrowly than the recording is generally used so as to eliminate the effect of adjacent tracks as much as possible.
The method can suppress the effect of the adjacent tracks to a minimum, but there is a problem in that since it is difficult to obtain sufficient reproducing output, ensuring sufficient SNR is difficult.
As one method for solving the heat fluctuation and ensuring the SNR or ensuring sufficient output, there has been an attempt to increase the track density by forming recessed and raised areas on the surface of the recording medium along the track to physically separate the recording tracks.
The technique is hereinafter referred to as a discrete track method, and a magnetic recording medium manufactured by the same is referred to as a discrete track medium.
As one example of the discrete track medium, there has been known a magnetic recording medium formed on a nonmagnetic substrate with a recessed and raised pattern formed on a surface thereof and having magnetic recording tracks and a servo signal pattern which are physically separated from each other (see Patent Document 1).
The magnetic recording medium has a ferromagnetic layer formed on the surface of the substrate with a plurality of recessed and raised areas formed on its surface, in which a soft magnetic layer is interposed between the surface of the substrate and the ferromagnetic layer, and a protective layer is formed on the surface of the ferromagnetic layer. The magnetic recording medium has a magnetic recording region formed in a raised area which is physically separated from its circumference.
With the magnetic recording medium, since the occurrence of a magnetic wall on the soft magnetic layer is suppressed, the effect of the heat fluctuation has hardly any effect. Further, since there is no interference between the adjacent signals, a high-density magnetic recording medium with little noise can be manufactured.
The discrete track method includes a method for forming a track after forming a magnetic recording medium made of several layers of thin films, and a method for forming a thin film of a magnetic recording medium after a recessed and raised pattern is previously directly formed on a surface of a substrate or is formed on a thin film layer for formation of the track (e.g., see Patent Document 2 and Patent Document 3). In these methods, the former is often referred to as a magnetic layer machining type, and the latter is referred to as an emboss machining type.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2004-164692
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2004-178793
[Patent Document 3] Japanese Unexamined Patent Application Publication No. 2004-178794
However, in the case in which the magnetic recording medium is used in the hard disk drive, there is a problem of corrosion resistance in the magnetic recording medium. That is, there is a problem in that since the magnetic recording medium used in the hard disk drive is made of a material containing Fe or Co, and the magnetic recording medium is easily corroded by water or the like introduced from the surroundings, thereby deteriorating the magnetization reversion characteristic of the magnetic recording medium or damaging the magnetic recording-reproducing head due to the corrosive substance attached to the magnetic recording-reproducing head.
The present invention takes the above circumstances into consideration, and an object thereof is to provide a magnetic recording medium capable of preventing a material containing Fe or Co from being easily corroded.